Sterki, V.

The Ohio Journal of Science

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April, 1911.] Anatomy and Physiology of the Unionidae. 33 rNOTES ON THE ANATOMY AND PHYSIOLOGY OF THEUNIONIDAE.*V. STERKI.The Unionidae are not only the most conspicuous part of ourmolluscan fauna, but also the most interesting. Some of theiranatomical and physiological features have come to our knowl-edge only recently, and in few groups of animals, have the last tento fifteen years brought such radical changes of classification.Up to 1900, the genera were generally based upon the shells: thosewith complete (regarding the family) hinges were called Unio,those with more or less defective hinges were Alasmidonta, orMargaritana, and those without hinge teeth wTere Anodonta. Con-chologists generally know that the groups and genera are nowestablished principally on the soft parts, mainly the branchiae,not exactly coincident with the formation of the shells. Thebranchiae, or gills, of this group of mollusca, have three verydifferent functions: respiration, nutrition (as food gatherers), andas brood chambers for the ova and embryos.The general morphology and anatomy of the fresh-watermussels is well known, but the special features are frequently notmentioned, or very fragmentarily, in text books on zoology, andnot even in recent special works on mollusca. Our lowest form,at least in one group, and in one direction, the small Anodontaimbecillis Say, is hermaphrodite, that is: part of the gonad isovary, another is testis. Other Anodontae have not been suffi-ciently examined in this respect. The balance of our Unionidaeare typically unisexual, yet among Quadrula, and even Lampsilis(parva, Barnes), bisexual individuals are occasionally found.* Presented at the Akron meeting of the Ohio Academy of Science,November 25, 1910.3 3 2 . The Ohio Naturalist. [Vol. XI, No. 6,It is known that the ova, from the ovary, pass through anoviduct on each side into the branchiae, where they develop intoembryos, the so-called glochidia. The glochidium, of about thesize of the ovum, has a two-valved shell, very different from thepostembryonal shell, and also of markedly different formation inthe several groups, and a very primitive formation of the softparts, without alimentary canal, ganglia, branchiae, etc.The formation of the female reproductive branchiae is variedand furnishes principal characters for classification. In some ofthe groups, the Unioninae (Unio, Pleurobema, Quadrula), also theAnodontinae (Anodonta, Alasmidonta, Gymphynota, etc.), thebranchiae which receive the ova, in their whole extent, showonly slight and macroscopically barely noticeable differences fromthe male branchiae, and the non-receptive of the female. In astill higher group, only a part of each of the outer branchiae isnoticeably differentiated, the so-called marsupium, consisting ofovisacs, their number being very different in the several groups,and approximately constant in adult individuals of each species.Also their configuration shows differences, when barren, and muchmore so when charged. This is the group, or subfamily Lamp-silinae, and, with some differences, Proptera. In Ptychobranchus(e. g, phaseolus Hildreth), the outer branchiae are differentiated intheir whole extent, and of a formation markedly different fromthat of the others, when gravid.In the lower forms, there are no or slightly marked differencesof the shells between males and females. With the appearance ofthe marsupium which, when filled and distended, projects moreor less over the general contour and the lower edge of the branchiae,there comes a corresponding distension of the shell in the female,not or slightly marked in some forms, strongly so in others, e. g.,most of the species of Lampsilis. It reaches its highest grade inTruncilla, where that part of the female shell is not only greatlydistended but also of a formation and sculpture different fromthe rest of the mussel.These differences, gradations, of both soft parts and shell, arenaturally not in a straight line, the same as in other groups ofanimals, but with ramifications and gaps, which latter wouldprobably be bridged over by extinct forms, and possibly by suchas are living in other zoo-geographical provinces.In connection with the different formation of the gravidbranchiae, there are also different ways of discharging the embryos.In the Unioninae the young are expelled upward from the broodchambers into the suprabranchial canal and from there out intothe water through the anal siphonal mantle opening. But in theLampsilinae, each ovisac opens, at its inferior end, andthe contents, coherent as a cake ("placenta"), makes its exitthrough that rent, and out either through the branchial siphonalApril, 1911.] Anatomy and Physiology of the Unionidae. 333opening, or simply through the great slit of the mantle on theventral side.Of the first stages of post-embryonal development, we stillknow little. It has been observed, in Europe, many years ago,that the glochidia of Anodonta attached themselves on fins, gills,etc., of fishes, are there inclosed in a cyst-like cavity by localhypertrophy of the host's epidermis or epithelium, and liveas parasites for weeks or months. In our country, some observa-tions of this kind have been made, but I have not seen a report onthem. There is an excellent opportunity here for observationsand experiments.At a later stage, small mussels—some less than two milli-meters long—are found with post-embryonal shells, still bearingthe glochidium valves in the centers of the beaks. Young Lamp-silis develop a byssus thread, about the thickness of a horse'hair, and several inches long, fastened to a stone, or shell. Theyoung mussel begins to develop its gonad in about the third year,and at that age has comparatively few ova and young in itsmarsupia. Only from that age on, young Lampsilinae beginto show sexual differences of the shells.There is another physiological feature of interest. By examin-ing thousands of specimens at various seasons of many years, ithas been found that the mussels of the several groups are producingtheir young at different times. The Unioninae, also Margaritana,are found with their branchiae barren through autumn, winterand spring, but ova, and sperms developed in the gonads. In thesummer, about June, the ova are transferred to the branchiaedevelop into glochidia within a week or two, and the young aredischarged soon; the whole process taking about four weeks. Inthe Lampsilinae, and the Anodontinae, the marsupia becomegravid in fall, in some as early as August; the transformation intoglochidia here also takes only a week or two, and then the embryos,without any noticeable changes, are retained over winter andearly spring, that is for eight to even ten months. The formerwere called short period or summer breeders, the latter longperiod or winter breeders.To sum up: From these condensed and fragmentary outlines,it becomes evident that our Unionidae are not of the simple anduniform organization as was supposed, and that their studyreveals many interesting features. For these reasons, they welldeserve more attention than has been given them, as an object ofstudy in the zoological laboratory, for their morphology, anatomyand physiology.In conclusion, it may not be amiss to point out briefly the princi-ple differences between the two groups of our fresh water Pelecypoda:the Unionidae of the Naiadacea, and the Sphaeriidae (Sphaerium,Mucsulirti; Pisidium and Eupera) of the Cyrenacea. The latter,334 The Ohio Naturalist. [Vol. XI, No. 6,of which we have about a hundred species, now known, in NorthAmerica, and well worth being studied, are of much smaller size,the mussels being 1.5 to 20 mill, long when mature; their hingesare more complete; the mantle is less open and the siphons areclosed, and tubes; the four branchiae are differently arranged; theyoung are developed in a special, brood pouch on the inside ofthe inner branchiae on each side; the young, when mature, aremuch larger than the glochidia of the Unionidae and fullydeveloped.New Philadelphia, Ohio.

Notes on the Anatomy and Physiology of the Unionidae

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Notes on the Anatomy and Physiology of the Unionidae

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